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“Internet questions (and answers) reveal motor misunderstandings" With
trilingual summary
For anyone with a question, the Internet has
become the place to seek answers. Several open forum or message board
web sites feature postings dealing with electrical technology
including motors and control. The questions being asked though
legitimate, are sometimes surprising in the ignorance they reveal on
the part of engineers and technicians who could be expected to know
better. More surprising are the misunderstandings apparent from the
answers being given.
New technology or changes in standard practices
are seldom involved. Rather, what's apparent is that misconceptions
about basic motor behavior remain widespread.
For example, a-c motor specifiers and users often
have questions about "starting current." Standards define no such
term. The current drawn by an accelerating motor takes on several
different values during the starting period, rather than being
constant as is often assumed.
A motor develops the same torque during
acceleration regardless of the load being accelerated. Use of a
"soft" or reduced voltage starting method offers several benefits,
but they do not include any reduction in thermal stress within the
motor (the reverse is often true) or any significant energy saving.
If a motor's accelerating torque is inadequate to start the load,
specifying a high torque NEMA Design C motor can be self-defeating,
because such a motor often has less thermal capacity for acceleration
heating than a standard design. The NEMA D alternative has that
disadvantage plus low efficiency at full speed. Simply oversizing the
motor means higher current -- and greater voltage drop -- during
starting.
Also widely misunderstood is motor power factor,
which is unaffected by the driven load; by the power factor in the
rest of the plant; or by the power factor on the utility feeder.
Concern is sometimes expressed that "overcorrection" can boost motor
power factor above 100 percent, which is a physical impossibility.
Many other misunderstandings that continue to
hamper proper motor selection and application can readily be found on
Internet forums. Anyone using that source of information needs to
proceed with caution.
From “Internet
questions (and answers) reveal motor misunderstandings"
from the Electrical Apparatus
December 2006
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Barks Publications, Inc. All Rights Reserved.
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